1. Field of the Invention
This Invention relates to the use of a snubber capacitor in a switching circuit, and more particularly to the use of a snubber capacitor in an electronic switching power supply.
2. Background Information
A switching circuit often develops ringing because of stray capacitance and stray inductance. Abruptly switching power to a circuit with stray capacitance and stray inductance leads to electric oscillation at the natural resonant frequency of the capacitance and inductance. The oscillation is referred to as xe2x80x9cringingxe2x80x9d in the circuit. Ringing usually occurs in a circuit when a switch supplying electric power to the circuit is either closed or opened.
In order to reduce ringing in a switching circuit, a capacitor in series with a resistor is often connected across the switch. The resistor dissipates the power of the oscillations, and the capacitor prevents a steady current flow through the resistor when voltage is applied across the resistor. The capacitor is referred to as a xe2x80x9csnubber capacitorxe2x80x9d, and the resistor is referred to as a xe2x80x9csnubber resistorxe2x80x9d.
A problem in a switching circuit in which the switch is closed and opened many times per second is that a considerable amount of power may be dissipated in the snubber resistor. Each time the switch is closed, the snubber capacitor charges up to the value of the applied DC voltage. Then, when the switch is opened, the snubber capacitor discharges through the snubber resistor, and perhaps through the stray capacitance. The charge stored in the capacitor is given by the product CV, where C is the capacitance and V is the voltage to which the capacitance is charged. Thus, on each closure of the switch, an amount of charge equal to CV is wasted. The amount of energy stored in a capacitor is given by xc2xd CV2. Thus, this amount of energy is wasted each time that the switch is closed and then opened.
The amount of energy wasted in a snubber capacitor becomes of concern when the switch is opened and closed many times per second. For example, in a switching power supply used for DC to DC conversion, the switches may operate in the frequency range of 100 kilohertz to 1000 kilohertz, or higher. Such rapid opening and closing of the switches, and consequently such rapid charging and discharging of the snubber capacitor, causes a significant waste of power through discharge of the snubber capacitor many times per second.
There is needed a method of suppressing oscillations in an electric circuit which does not waste considerable amounts of power during operation of a switch, especially when the switch rapidly closes and opens.
The invention replaces the snubber resistor with a two diodes and a load. The electric charge stored in the snubber capacitor on each cycle of the switch is then caused by a first diode to flow through the load, rather than to flow through a snubber resistor. A second diode provides a charging path for the snubber capacitor. The load uses the electric power which would be wasted in the snubber resistor.
In an aspect of the invention, two sets of snubber capacitors and diode pairs are used to reduce ringing in a synchronous switching power supply. For example, field effect transistors (FETs) are used to switch the primary of a transformer in a switching DC to DC converter, and a snubber capacitor along with a pair of diodes is used on each end of the primary winding of the transformer to both reduce ringing in the primary circuit, and to supply power from electric charge stored in the snubber capacitors. For example, the power recovered from the snubber capacitors may be supplied to control circuits which operate the DC to DC converter, etc.